The manufacture of wood products from wood fibers or chips, such as fiberboard, particle board and oriented strand board (OSB) is well known. Typically, the wood fibers or chips are mixed with a suitable adhesive and wax emulsion and the mixture is then matted and pressed under high pressure and temperature to form a rigid, dense panel. OSB is manufactured using chips or strands sliced from logs in the orientation of the grain. Typically the strands are 4-6 inches in length and 1 inch wide and have a uniform thickness. After drying and sorting, the strands are mixed with the wax and adhesive and oriented in layers. The strands in the layers which will form the exterior surface of the panel are aligned in the long direction of the panel while the inner layers are cross-aligned to the surface layers.
The adhesives commonly employed in the manufacture of wood products from wood fibers or chips include phenolic resins or isocyanate binders. Phenolic resins, produced by reacting phenol with formaldehyde under alkaline conditions have been used for many years. Recently, there have been concerns raised about the production, use and handling of such resins and a number of manufacturers have switched to using isocyanate binders. Liquid polymeric dimethyl phenate di-isocyanate (pMDI) is now becoming the binder of choice. pMDI is an excellent adhesive and provide advantages for the board over other adhesives. However, pMDI provides adhesion not only for the wood fibers, but also for the boards to the metal press platens, caul plates or stainless steel screens used in the manufacturing process. Therefore, many producers of wood products manufactured from wood fibers or chips will use a surface layer of non-pMDI bonded wood fibers or chips, such as phenol formaldehyde resin, and use pMDI in the core layer of the board to prevent the adhesion between pMDI in the mat and the metal surfaces. This complicated surface-core sandwiching process has increased the cost of board manufacturing and has prevented the industry from benefiting fully from pMDI adhesive.
There have been attempts to overcome the above bonding to the metal surfaces through the use of release agents. Many conventional release agents as used in the industry do not provide satisfactory release. Some of these require elaborate and costly processes for pre-treatment of the press platens, such as applying internal and external release agents, multiple coatings and baking the platens. Other release agents can provide good release, but must be applied in high concentration of up to 80% and require much lower press temperatures and longer press time, thus increasing production time and cost. Some release agents based on higher surface active materials such as silicones may remain associated with the finished board surface and thus affect the paintability of the board.
We have previously developed a release agent composition for use with a pMDI adhesive comprising a mixture of an alkali metal salt of a fatty acid having at least 8 carbon atoms and an alkali metal salt of an ethoxylated phosphate ester having 8-16 carbon atoms. This formulation was a solution marketed as PRESSGUARD™ (Guardian Chemicals Inc.) and had certain advantages when sprayed on the strand mat or caul plate using spray bar systems, which are commonly used in North America. However, when it was attempted to apply the release agent by other methods which agitate the release agent, such as spinning disc spray or rotating roller systems, which are popular in Europe and Asia, it was found that the agitation and turbulence of those methods caused unacceptable foaming of the release agent. This tendency to foam impacted the release agent application rate resulting in reduced coverage and slower performance.
In applications where foaming is encountered, the traditional solution is the use of a de-foaming agent. There are however problems with this approach. De-foaming agents tend not to be soluble in water-based solutions and need added hydrotropes to increase their solubility. De-foaming agents in such solutions tend not to be stable, especially on storage or if the solution is diluted for final application. On dilution, the effect of the hydrotrope is reduced and the de-foaming agent may come out of solution, leading to separation and loss of de-foaming properties. One solution to these problems has been the application in a separate step of a de-foaming agent. However, this requires additional equipment for the de-foaming agent to be separately applied. In addition, many de-foaming agents are silicone-based which can cause problems with paintability of the final board.
Thus there still remains a need for a pMDI release agent which has significantly reduced or eliminated foaming tendencies and will provide effective release between the board and the metal surfaces in conventional processes using conventional equipment at conventional speeds.